This invention relates generally to condition-responsive switches and, more particularly, to a defrost on demand thermostat for deicing an evaporator surface.
Switches that are responsive to temperature changes, commonly known as thermostats or cold controls, are used in refrigeration systems, such as refrigerators, freezers, air conditioning systems, and heat pumps for controlling the temperature of air in a compartment, room, building, etc. These thermostats regulate the switching cycle of a compressor in response to the temperature of the air contained at a remote location. When the temperature exceeds a certain xe2x80x9cturn-onxe2x80x9d point, the switch contacts are closed and the compressor is switched on to cool the air. When the temperature drops below a certain xe2x80x9cturn-offxe2x80x9d point, the switch contacts are opened and the compressor is switched off.
The compressor receives a refrigerant from an evaporator, which typically operates at sub-freezing temperatures. Therefore, the surface of the evaporator often becomes frosted or covered with ice, which insulates the evaporator and prevents it from absorbing heat, thereby negatively affecting the efficiency of the refrigerator system. From time to time, the evaporator may be defrosted according to known methods, but typically only after long periods of inefficient use because of ice build-up on the evaporator. In addition, if ice is allowed to build-up on the evaporator, the defrost cycle time increases. Also, temporary relief of conventional evaporator defrost is usually short-lived as the evaporator quickly becomes frosted again. While separately controlled defrost mechanisms exist, they increase the costs of the system as well as complicate assembly.
Accordingly, it would be desirable to provide a thermostat that controls the application of a defrost cycle to defrost the evaporator surface as needed, or on demand, to prevent excessive ice buildup on the evaporator surface, to reduce the costs of separate switches to control a defrost mechanism, and to simplify the assembly of refrigeration systems.
In an exemplary embodiment of the invention, a condition responsive electric switch includes a housing, an actuator arm, a movable electrical contact, and an adjustable electrical contact for completing a defrost circuit. The actuator arm is mounted within the housing for pivotal movement that moves the movable contact and opens and closes the movable and adjustable electrical contacts to complete or to break a defrost circuit through the switch in response to the temperature of an evaporator tube connected to an evaporator.
When the evaporator tube ices over, the temperature of the evaporator tube drops, causing a refrigerant inside a bellows and in fluid communication with the evaporator tube to contract and move the actuator arm, which causes the movable contact to engage the adjustable contact and complete a circuit through a defrost mechanism known in the art. As the temperature of the evaporator tube rises due to defrost of the evaporator, the refrigerant expands in the bellows and causes the actuator arm to move and disengage the movable contact from the adjustable contact. Thus, an automatic defrost on demand switch control is provided that defrosts an evaporator on an as needed basis, making the system more efficient.
The position of the adjustable contact relative to the movable contact is adjustable to calibrate the sensitivity of the defrost circuit to movement of the actuator arm. Thus, the defrost contact may be moved closer to or farther away from the movable contact to vary the required temperature change, or differential in the evaporator tube temperature to engage the movable contact and the defrost contact and close the defrost circuit. The smaller the separation of the movable contact and the defrost contact, the smaller the temperature differential in the evaporator tube that will close the contacts.